The conditions responsible for modulation of in vitro transformation are being investigated, with specific emphasis of the use of UV, X-rays, combinations of carcinogens of different classes, the combination of in vivo - in vitro systems, and the effect of repair mechanisms on toxicity and transformation. The relevance of chromosome damage as a result of initial carcinogen insult, as well as to which cytogenetic alterations are responsible for or associated with malignancy is being determined using cells from a variety of species including human. Studies on the mechanisms of early events of carcinogen cell interaction and investigations of different proximate carcinogens and metabolic inhibitors are emphasized. The requirements necessary to obtain reproducible neoplastic transformation of human fetal fibroblasts and adult epithelial cells by chemical carcinogens are being determined. The testing for the tumorigenicity of cells exposed to different agents for different time intervals will continue.